Yes, Jerick McKinnon is just a rookie and yes he plays the same position as Adrian Peterson, the best running back in the NFL. Even still, McKinnon looks to be a player who will contribute to the Vikings offense immediately. He was billed as a scat-back type who could contribute on 3rd downs as a receiver, and that is true, he can do that. But he has also displayed some inside running ability and the ability to perform on special teams in a variety of capacities. The Vikings found a gem in the 3rd round with McKinnon.
It's not clear who the proper handcuff is for Peterson owners. Most likely, the team would committee Matt Asiata with McKinnon if something were to happen to Peterson.